Integrated Circuits (ICs) generally comprise many semiconductor features, such as transistors, formed on a semiconductor substrate. The patterns used to form the devices may be defined using a process known as photolithography. Using photolithography, light is shone through a pattern on a mask, transferring the pattern to a layer of photoresist on the semiconductor substrate. The photoresist can then be developed, removing the exposed photoresist and leaving the pattern on the substrate. Various other techniques, such as ion implantation, etching, etc. can then be performed to the exposed portion of the substrate to form the individual devices.
To increase the speed of ICs such as microprocessors, more and more transistors are added to the ICs. Therefore, the size of the individual devices must be reduced. One way to reduce the size of individual features is to use short wavelength light during the photolithography process. According to Raleigh's Law (R=k*λ/NA, where k is a constant, and NA=Numerical Aperture, and R is the resolution of features), a reduction in the wavelength of the light proportionately reduces the size of printed features.
Extreme ultraviolet (EUV) light (13.5 nm) is now being used to print very small semiconductor features. For example, EUV can be used to print isolated features that are 15–20 nanometers (nm) in length, and nested features and group structures that have 50 nm lines and spaces. EUV lithography is targeted to meet the requirements of a 50 nm half-pitch, where pitch is equal to line plus feature size. Since EUV light has such a short wavelength, it is easily absorbed, even by air. Therefore, EUV photolithography is performed in a vacuum using multilayer-coated reflective optics.
EUV photons can be generated by the excited the atoms of a plasma. One way to generate the plasma is to project a laser beam on to a target (droplet, filament jet) creating a highly dense plasma. When the excited atoms of the plasma return to a stable state, photons of a certain energy, and thereby a certain wavelength, are emitted. The target may be, for example, Xenon, Tin, or Lithium. Another way to produce EUV photons is to create a pinch plasma between two electrodes with the target material in a gaseous form between the two electrodes, thereby exciting the atoms.
When very coherent light is used with EUV optics, there tends to be significant ringing in the diffraction order. As a result, partially coherent light is used for EUV lithography. The partial coherence of photolithography system is defined by NAill/NApro, where NAill is the numerical aperture on the illumination side of the optics, and NApro is the numerical aperture on the projection side of the optics. The numerical aperture is the size of the orifice through which light passes. A measure of partial coherence is often referred to as a “sigma” or “σ” value.
Different partial coherence values may be used for different types of features. For example, for printing isolated lines, is advantageous to use smaller partial coherence values. For tighter pitches (denser printing), higher partial coherence values result in increased resolution. Therefore, adjustable apertures have been used with standard wavelength lithography to adjust the partial coherence of the light. These adjustable apertures may be zoomable lenses or mirrors. However, zoom lenses for reflective optics are extremely complex and very lossy due to multiple reflections (for example, at each bounce, 30% of the light may be lost).
Typically, when zoomable mirrors are not present, a user begins with one set partial coherence value. The numerical aperture on the illumination side is designed to be as large as possible, and then the aperture on the illumination side is stepped down to change the numerical aperture on the illumination side. While this technique gives a wide range of partial coherence values, it significantly reduces the intensity of transmitted light.